1. Field of the Invention
The present invention relates to an H-bridge motor driving circuit for controlling a DC motor using pulse width modulation (PWM).
2. Description of the Related Art
One general motor driving circuit for controlling a DC motor to rotate selectively in normal and reverse directions is an H-bridge motor driving circuit which has an H-shaped bridge circuit comprising four transistors and a DC motor. The four transistors are turned on and off to energize, de-energize, and rotate the DC motor selectively in the normal and reverse directions.
FIG. 1 of the accompanying drawings shows a typical conventional H-bridge motor driving circuit.
As shown in FIG. 1, the conventional H-bridge motor driving circuit comprises H-bridge output circuit 10, triangular wave oscillator 11, PWM comparator 12, inverter 13, and control circuit 14.
H-bridge output circuit 10 has MOS transistors Q1, Q2 each having a drain connected to the positive terminal of a DC power supply E, a source connected to the circuit board, and a gate supplied with control signals S11, S12, respectively, for turning on and off MOS transistors Q1, Q2, and MOS transistors Q3, Q4, each having a drain connected to the sources of MOS transistors Q1, Q2, respectively, a source connected to the circuit boards and a ground potential point to which the negative terminal of the DC power supply E is connected, and a gate supplied with control signals S13, S14, respectively, for turning on and off MOS transistors Q3, Q4. Motor M is connected between the junction between MOS transistors Q1, Q3 and the junction between MOS transistors Q2, Q4. Parasitic diodes D1 through D4 exist at the junctions between the sources of MOS transistors Q1 through Q4, the circuit board, and the drains of MOS transistors Q1 through Q4.
Triangular wave oscillator 11 generates triangular wave signal S21.
PWM comparator 12 compares triangular wave signal S21 from triangular wave oscillator 11 with constant-level signal S22, and outputs PWM pulse signal S23.
Inverter 13 inverts pulse signal S23 from PWM comparator 12 into pulse signal S24.
Control circuit 14 is supplied with pulse signals S23, S24, power-supply-level signals S25, S26, and rotation control signal S0, and outputs signals S25, S24, S26, S23 as control signals S11 through S14 for MOS transistors Q1 through Q4.
Operation of the conventional H-bridge motor driving circuit shown in FIG. 1 will be described below with reference to a timing chart of FIG. 4 of the accompanying drawings.
Triangular wave signal S21 generated by triangular wave oscillator 11 and constant-level signal S22 are supplied to comparator 12, which generates PWM pulse signal S23. PWM pulse signal S23 is inverted into signal S24 by inverter 13. Signals S23, S24, power-supply-level signal S25. and ground-level signal S26 are supplied to control circuit 14, and then applied as respective control signals S14, S12, S11, S13 to the gates of MOS transistors Q4, Q2, Q1, Q3, respectively. It is assumed that MOS transistors Q1, Q4 are energized , and MOS transistor Q4 is PWM-controlled. During period T1, signals S23, S25 are high and MOS transistors Q1, Q4 are turned on, causing a current to flow through motor M. During period T2, signals S24, S25 are high and MOS transistors Q1, Q2 are turned on, entering a regenerative mode to produce a regenerative current flowing through a loop from motor M to MOS transistor Q2 to MOS transistor Q1 to motor M (in case of an inductive load). In case of a resistive load, the MOS transistors are not conducted.
The conventional H-bridge motor driving circuit described above is disadvantageous in that since MOS transistor Q1 is energized at all times and hence a current flows through MOS transistor Q1 at all times, a large amount of electric power needs to be supplied to the H-bridge motor driving circuit, which generates a large amount of heat and hence suffers poor reliability. The H-bridge motor driving circuit is necessarily of increased cost as it needs a high-performance device such as a low-on-resistance MOSFET or a low-saturation-voltage transistor for reducing the amount of generated heat.
It is an object of the present invention to provide an H-bridge motor driving circuit which includes a circuit arrangement for distributing an amount of applied electric power among a plurality of transistors to equalize the amounts of heat generated by the transistors.
To achieve the above object, an H-bridge motor driving circuit according to the present invention comprises, connected between a PWM comparator and a control circuit, first and second frequency dividers for frequency-dividing, by 2, an output signal from the PWM comparator with positive-going edges and negative-going edges, respectively, thereof, an AND gate for ANDing an output signal from the first frequency divider, an OR gate for ORing the output signal from the first frequency divider and the output signal from the second frequency divider, and first and second inverters for inverting an output signal from the AND gate and an output signal from the OR gate, respectively.
The control circuit applies an output signal from the first inverter, an output signal from the second inverter, the output signal from the AND gate, and the output signal from the OR gate to the gates of first, second, third, and fourth MOS transistors, respectively.
During a first period, the first and second transistors are turned on, and the second and third transistors are turned off, causing a current to flow through a motor. During a next second period, the third and fourth transistors are turned on, and the first and second transistors are turned off, causing a regenerative current to flow through the motor. During a next third period, the MOS transistors are turned on and off in the same manner as during the first period, causing a current to flow through the motor. During a final fourth period, the first and second transistors are turned on, and the third and fourth transistors are turned off, causing a regenerative current to flow through the motor.
In a regenerative mode, the first and second MOS transistors, and the third and fourth MOS transistors are alternately turned on. Therefore, the amount of applied electric power is distributed among the MOS transistors, thus equalizing the amounts of heat generated by the MOS transistors.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference t the accompanying drawing s which illustrate an example of the present invention.